Direct Arylation Synthesis of Small Molecular Acceptors for Organic Solar Cells

In recent years, small molecular acceptors (SMAs) have extensively promoted the progress of organic solar cells (OSCs). The facile tuning of chemical structures affords SMAs excellent tunability of their absorption and energy levels, and it gives SMA-based OSCs slight energy loss, enabling OSCs to achieve high power conversion efficiencies (e.g., >18%). However, SMAs always suffer complicated chemical structures requiring multiple-step synthesis and cumbersome purification, which is unfavorable to the large-scale production of SMAs and OSC devices for industrialization. Direct arylation coupling reaction via aromatic C-H bonds activation allows for the synthesis of SMAs under mild conditions, and it simultaneously reduces synthetic steps, synthetic difficulty, and toxic by-products. This review provides an overview of the progress of SMA synthesis through direct arylation and summarizes the typical reaction conditions to highlight the field’s challenges. Significantly, the impacts of direct arylation conditions on reaction activity and reaction yield of the different reactants’ structures are discussed and highlighted. This review gives a comprehensive view of preparing SMAs by direct arylation reactions to cause attention to the facile and low-cost synthesis of photovoltaic materials for OSCs.


Introduction
Organic solar cells (OSCs) have generated significant interest in both research and industrial communities due to their inherent advantages. Compared with other types of solar cells [1][2][3][4], OSCs show the features of tunable optoelectrical properties, high flexibility, lightweight, and facile large-area devices fabrication through low-cost solutionprocessing techniques [5][6][7]. In recent years, small-molecule acceptors (SMAs) significantly promote the development of OSCs and improve the power conversion efficiency (PCE) of OSCs [8][9][10][11][12]. SMAs have been proven to be the most efficient electron-accepting materials for OSCs. Moreover, SMA-based OSCs have realized PCEs of over 18% [13][14][15][16][17][18][19]. However, the present high-performance SMAs [4,5] with complicated molecular structures usually require multiple-step synthesis and cumbersome purification which results in low synthetic yield, high cost, and difficult large-scale production. Hence, designing efficient SMAs with simple structures and reducing the synthesis steps and complexity become one of the most important topics for the development of OSCs. Nearly all strategies to synthesize high-efficiency SMAs involve Stille-coupling reactions to form aryl-aryl bonds to construct π-conjugated structures. However, the Stille-coupling reaction requires the preactivation of the reactant by introducing costly organotin moieties through highly reactive organometallic reagents (mainly n-butyl lithium) under strictly anhydrous and oxygen-free conditions at low temperatures (e.g., −78 • C). In addition, the process of producing Stille coupling products is accompanied by the formation of stoichiometric quantities of toxic trialkyl-stannanes by-products. Obviously, the excessive synthesis steps at strict conditions, cumbersome purification, and poisonous by-products will lead to severe synthetic difficulty and cost. At the same time, the pre-activation and toxic wastes tend to induce safety and environmental concerns.
In contrast, direct arylation coupling reaction directly uses aryl C-H bonds of (hetero) aryl derivatives as coupling partners to generate aryl-aryl structures [20][21][22][23][24][25], which gets rid of the pre-functionalization of (hetero)arenes. This strategy simplifies the preparation and purification, improves atom economy, and generates only environmental-benign wastes. These advantages are favorable to decreasing the synthetic difficulty and overall cost of π-conjugated molecules. Therefore, the direct arylation coupling reaction benefits the preparation of π-conjugated polymers and small molecules by reducing reaction steps and simplifying the synthetic processes.
Direct arylation coupling has been studied intensively in the past few years. It has been demonstrated as a promising and practical approach in preparing π-conjugated materials for OSCs [26][27][28], which is conducive to the development and application of OSCs. Without concern for structure defect residues, the production of small molecule semiconductors and monomers for π-conjugated polymers should more readily benefit from the convenient and straightforward direct arylation coupling reaction. However, the research and development of the synthesis of SMAs by direct arylation reactions were far inferior to that of conjugated polymer photovoltaic materials [27,[29][30][31][32]. The present review aims to summarize the progress of the synthesis of SMAs by direct arylation and highlight the influence of reaction conditions of direct arylation on the reactivity and selectivity of typical building blocks in SMAs.

Synthesis of Small-Molecule Acceptors
In 2016, Chen et al. reported the synthesis of acceptors SMA1 by the direct arylation coupling reaction [33]. As shown in Figure 1, the electron-withdrawing group thiophene-2-carbonitrile (2) was connected to mono-brominated thiophene-diketopyrrolopyrrole (DPP) 1 through direct arylation coupling reaction under the catalyst of palladium(II) acetate (Pd(OAc) 2 ). However, because of the competition of the self-coupling reaction of 1, compound 3 was obtained at a low yield of 24%. After bromination, the intermediate compound 4 further reacted with fluorene diboronic ester (5) through Suzuki coupling to afford the final acceptor SMA1 in 70% yield. SMA1 molecules exhibit a high electron mobility of up to 10 −3 cm 2 V −1 s −1 in bulk heterojunction blends. OSCs employing SMA1 as an acceptor and poly (3-hexylthiophene) (P3HT) as a donor showed a PCE of 2.37% with an open circuit voltage (V oc ) of 0.97 V, a short circuit current density (J sc ) of 6.25 mA cm −2 , and a fill factor (FF) of 0.39. In 2017, an unsymmetrical acceptor SMA2, consisting of N-annulated perylene diimide (PDI) (6), thienyl DPP (7), and indoloquinoxaline (9) units to enable panchromatic absorption, was synthesized by two-step direct arylation coupling reactions ( Figure 2) [34]. In the beginning, 6 reacted with 7 catalyzed with the heterogeneous palladium In 2017, an unsymmetrical acceptor SMA2, consisting of N-annulated perylene diimide (PDI) (6), thienyl DPP (7), and indoloquinoxaline (9) units to enable panchromatic absorption, was synthesized by two-step direct arylation coupling reactions ( Figure 2) [34]. In the beginning, 6 reacted with 7 catalyzed with the heterogeneous palladium catalyst SiliaCat ® DPP-Pd, to obtain the mono-substituted product 8 in 45% yield. Moreover, the following direct arylation coupling reaction was performed between 8 and 9 in the same condition but higher reaction temperature of 90 • C to form the acceptor SMA2 with a satisfactory yield (85%). However, OSCs taking SMA2 as the acceptor and P3HT as the donor showed a relatively low PCE of 0.81%. Then, a series of symmetrical acceptors SMA3-SMA9 flanked by N-annulated PDI were further prepared [35][36][37][38][39]. Chemical structures and synthetic routes of the acceptors SMA3-SMA9 are shown in Figure 3. PDI monobromide 6 could react with the unactivated thiophene-containing counterparts in the presence of SiliaCat ® DPP-Pd, potassium carbonate (K 2 CO 3 ), and pivalic acid (PivOH) in N,N-dimethylacetamide (DMAc) to obtain the target products SMA3-SMA9 in moderate yields (51-74%). In 2017, an unsymmetrical acceptor SMA2, consisting of N-annulated perylene diimide (PDI) (6), thienyl DPP (7), and indoloquinoxaline (9) units to enable panchromatic absorption, was synthesized by two-step direct arylation coupling reactions ( Figure 2) [34]. In the beginning, 6 reacted with 7 catalyzed with the heterogeneous palladium catalyst SiliaCat ® DPP-Pd, to obtain the mono-substituted product 8 in 45% yield. Moreover, the following direct arylation coupling reaction was performed between 8 and 9 in the same condition but higher reaction temperature of 90 °C to form the acceptor SMA2 with a satisfactory yield (85%). However, OSCs taking SMA2 as the acceptor and P3HT as the donor showed a relatively low PCE of 0.81%. Then, a series of symmetrical acceptors SMA3-SMA9 flanked by N-annulated PDI were further prepared [35,36,37,38,39]. Chemical structures and synthetic routes of the acceptors SMA3-SMA9 are shown in Figure 3. PDI monobromide 6 could react with the unactivated thiophenecontaining counterparts in the presence of SiliaCat ® DPP-Pd, potassium carbonate (K2CO3), and pivalic acid (PivOH) in N,N-dimethylacetamide (DMAc) to obtain the target products SMA3-SMA9 in moderate yields (51-74%).  Firstly, the thiophene-and bithiophene-bridged compounds SMA3 and SMA4 were synthesized by direct heteroarylation [35]. Using a combination of 6 and thiophene-PDI synthon 10, SMA3 was synthesized with a yield of 70%. While the bithiophene 11  Firstly, the thiophene-and bithiophene-bridged compounds SMA3 and SMA4 were synthesized by direct heteroarylation [35]. Using a combination of 6 and thiophene-PDI synthon 10, SMA3 was synthesized with a yield of 70%. While the bithiophene 11 coupling partner was utilized to react with 6 by double direct arylation coupling to form small molecule SMA4 in 51% yield. The lowest unoccupied molecular orbital (LUMO) energy levels of the acceptors were relatively high, which was beneficial to obtain the high V oc of the corresponding OSCs. As a result, the V oc of the OSCs using PTB7-Th as the donor and SMA3 and SMA4 as the acceptor reached 0.99 V and 1.05 V, respectively. However, due to the lack of tendency for the acceptors to form crystalline domains, the blend films using SMA3 and SMA4 as acceptors and PTB7-Th as donors presented poor morphology, which resulted in low J sc and low fill factors for the corresponding OSCs. Finally, the PCE of the OSCs based on PTB7-Th:SMA3 and PTB7-Th:SMA4 were only 2.02% and 2.74%, respectively (Table 1).  [39] Bithiophene is one of the most commonly used building blocks in organic semiconductors because of its electronic properties, easy preparation, and high stability. More importantly, the properties of bithiophene-based materials are readily tunable through the functionalization of the building blocks. The novel way to develop bithiophene derivative building blocks is to incorporate a bridge unit at the 3-and 3 -positions to form a fused five-or six-membered ring system or to insert a π-conjugated unit between the two thiophenes to generate a large π-conjugated system. Thus, a bithiophene derivative dithienophosphole oxide (12) was exploited as the central core to synthesize the new acceptor SMA5 (Figure 3) [36]. Owing to the high sensitivity of the phosphole-containing structure under hot and basic conditions, the direct arylation reaction was carefully optimized to restrain the decomposition of the dithienophosphole oxide unit. Employing the optimized conditions (SiliaCats ® DPP-Pd, K 2 CO 3 , and PivOH in DMAc, 80 • C), SMA5 was obtained in 28% yield through bead bath and in 49% yield through microwave heating. Compared with that of its phosphole-free analog SMA4, the phosphole unit could efficiently lower the energy levels of SMA5. As a result, using PTB7-Th as a donor, OSCs based on PTB7-Th:SMA5 showed a slightly decreased V oc (0.99 V) than that of SMA4 (1.05 V). Combined with the reduced J sc , the PCE of SMA5-based OSCs (2.05%) is slightly lower than that of SMA4 (2.74%). Nevertheless, all SMA5-based devices using different electron donors, including PTB7-Th, PBDB-T, and PDTT-BOBT, presented high V oc in the range of approximately 1.0~1.1 V, paving the way for the further design of SMAs for OSCs with high V oc .
To expand the design strategy of N-annulated PDI-flanked acceptors and obtain high photovoltaic performance based on the molecular skeleton of SMA4, new accep-tor molecules SMA6, SMA7, and SMA8, were prepared by incorporating isoindigo, thienoisoindigo, and DPP in the middle of bithiophene as shown in Figure 3 [37]. Moreover, these acceptors were prepared from N-annulated PDI monobromide 6 and bisthiophenesubstituted counterparts (13, 14, and 15) by direct arylation reaction in the presence of SiliaCat ® DPP-Pd, K 2 CO 3 , and PivOH in DMAc and the target products were isolated in appreciable yields of 67%, 61%, and 74% for SMA6, SMA7, and SMA8, respectively. Because of the nature of the dye core (i.e., isoindigo, thienoisoindigo, and DPP), SMA6, SMA7, and SMA8 displayed larger absorption coefficients and additional absorption peaks or shoulders in longer wavelength regions, compared to SMA4, which could benefit the absorption and utilization of light. PTB7-Th was used as an electron donor to evaluate the influence of the different dye cores on the photovoltaic performance of these acceptors. PCE reached 2.6%, 0.4%, and 2.9% for the SMA6-, SMA7-, and SMA8-based OSCs, respectively. This result indicated that thiophene-DPP was the superior bridge in this series of N-annulated PDI-flanked acceptors. At the same time, the DPP-embedded acceptor SMA8 was proved to be sufficiently amenable to the SiliaCats ® DPP-Pd catalyzed direct arylation coupling reaction, and it could be prepared with the highest yield.
The thiophene-DPP unit has been widely applied for organic optoelectronic devices. Moreover, thiophene-DPP 15 was synthesized from its un-alkylated precursor. Hence, it is easy to change the alkyl side chains attached to the nitrogen atoms of the DPP moiety to tune the assembly and aggregation properties of the molecules. Therefore, DPP-based material SMA9 with linear n-octyl substituents on the DPP core was readily prepared from 16 ( Figure 3) [38]. Compared to SMA8, SMA9 exhibited obvious thin-film re-organization upon taking 1,8-diiodooctane as the additive. This re-organization was beneficial for the J sc and FF enhancement of SMA9-based OSCs to accomplish an improved efficiency of 4.1%. Moreover, it was found that SMA9 displayed a favorable response to post-deposition solvent vapor annealing (SVA) conditions. The chloroform SVA treatment remarkably changed the absorption profile, film morphology, and charge carrier mobility of the SMA9based active layer, and the consequent photovoltaic performance of the corresponding OSCs. As a result, the PCE of OSCs based on PTB7-Th:SMA9 reached 5.6%, with a higher J sc of 11.32 mA cm −2 .
To meet the demands of widespread use and large-area OSC devices, a scale-up synthesis of SMA9 was carried out, and a screening of benzo[1,2-b:4,5-b ]dithiophene (BDT)-based donor polymers was investigated [39]. Taking advantage of the direct arylation cross-coupling protocol and the re-usable silica-supported palladium catalyst SiliaCats ® DPP-Pd, SMA9 was prepared with an isolated yield of 68% at a multi-gram scale. To the best of our knowledge, this was the largest scale-up synthesis of the acceptor materials by direct arylation reaction. The resulting SMA9 was paired with different BDT-based donor polymers, i.e., PTB7-Th, TTFQx-T1, PBDB-T, and J61, to construct OSCs, and PCE of the OSCs was in the range of 2.3-5.7%, as shown in Table 1. After interfacial modification, the PCE of solar cells based on PTB7-Th:SMA9 was improved to 6.2%, with a significantly increased J sc of 14.52 mA cm −2 .
In 2018, the SiliaCats ® DPP-Pd catalyzed direct arylation reaction was extended to synthesize an indacenodithieno[3,2-b]thiophene (IDTT) linked N-annulated PDI acceptor [40]. Surprisingly, instead of the bis-substituted product SMA10, the SiliaCats ® DPP-Pd catalyzed reaction proceeded largely to the tetra-substituted product SMA11 as shown in Figure 4. This result provided a new way to access the quadruple PDI compound. By increasing the feed ratio of 6 and 17 to 4.2 and the reaction temperature to 120 • C, SMA11 was obtained as a fine dark red crystalline powder in 70% yield. As expected, with four PDI units, SMA11 displayed a high molar absorption coefficient exceeding 200,000 M −1 cm −1 , approximately four times that of monomeric PDI species. OSCs taking the polymer PTB7-Th as a donor and SMA11 as an acceptor were fabricated to study the photovoltaic performances. The inverted bulk-heterojunction (BHJ) OSCs based on PTB7-Th:SMA11 showed a good photovoltaic response with PCEs of 3.4% and a high V oc of up to 1.0 V. The hole and electron mobilities of the PTB7-Th:SMA11-based active layer were 2.74 × 10 −4 cm 2 V −1 s −1 and 4.00 × 10 −6 cm 2 V −1 s −1 , respectively, through space charge-limited current (SCLC) measurement. The active layer's low electron mobility and unbalanced charge transport should be one of the most important reasons for the low J sc (8~10 mA cm −2 ) and FF (0.34~0.35). All in all, this study provided a new way for the synthesis and further development of tetra-substituted IDTT derivatives by C-H direct arylation.  Due to steric hindrance between the α-and β-substituents, the N-annulated PDI units were almost perpendicular to the IDTT core. Thus, there is minimal electron delocalization between the IDTT and PDI units. This contributed to the lack of prolonged wavelength absorption and low electron mobility of SMA11. Perhaps the α,α-bis-substituted PDI-IDTT product SMA10 with better planarity and conjugation between PDI and IDTT segments should make better use of sunlight and obtain higher electron mobility and improved photovoltaic performance. On the other hand, the formation of this product demonstrated that both α-C−H and β-C−H bonds in thiophenes of the IDTT core were simultaneously activated under the SiliaCats ® DPP-Pd catalyzed condition. In fact, the unselective C-H activation was considered the major drawback of direct arylation reactions, which decreased reaction yields in the synthesis of small molecules and caused structural defects in the synthesis of polymers. The β-defects (branching defects) severely degraded the photovoltaic properties of the resulting polymers and cannot be removed [41,42]. Besides the limited C−H bond selectivity, the heterogeneous palladium catalyst in strong a polar solvent DMAc also caused solubility issues in the resulting SMAs. As a result, SiliaCat ® DPP-Pd catalyzed direct arylation mainly generated the coupling products with a yield of not more than 70%.
Moreover, our previous studies showed that nonpolar solvent conditions could offer direct arylation with more excellent selectivity and higher reaction activity [43]. This approach was extended to synthesize an A-D-A trimer involving indacenodithienothiophene (IDT) derivatives to construct SMAs ( Figure 5). With the catalyst of tris(dibenzylideneacetone)dipalladium(0) (Pd2(dba)3)/tris(omethoxyphenyl)phosphine (P(o-MeOPh)3, L1), in the presence of K2CO3 and PivOH in 1,2dimethylbenzene(ODMB), 4-formyl benzotriazole (BTA) bromide 19 was coupled with IDT 18 to achieve the α,α-bis-(aldehyde benzotriazole) flanked IDT (20) in an excellent yield of 92%, with only trace α-mono-substituted product. In addition, no β-substituted (tris-and tetra-substituted) products were identified [44]. Based on the critical intermediate di-aldehyde 20, SMA12−SMA14 with different end groups were synthesized by a Knoevenagel condensation reaction. Using J52-F as the electron donor, the PCE reached 9.04%, 5.61%, and 11.27%, for SMA12-, SMA13-, and SMA14-based OSCs, Due to steric hindrance between the α-and β-substituents, the N-annulated PDI units were almost perpendicular to the IDTT core. Thus, there is minimal electron delocalization between the IDTT and PDI units. This contributed to the lack of prolonged wavelength absorption and low electron mobility of SMA11. Perhaps the α,α-bis-substituted PDI-IDTT product SMA10 with better planarity and conjugation between PDI and IDTT segments should make better use of sunlight and obtain higher electron mobility and improved photovoltaic performance. On the other hand, the formation of this product demonstrated that both α-C-H and β-C-H bonds in thiophenes of the IDTT core were simultaneously activated under the SiliaCats ® DPP-Pd catalyzed condition. In fact, the unselective C-H activation was considered the major drawback of direct arylation reactions, which decreased reaction yields in the synthesis of small molecules and caused structural defects in the synthesis of polymers. The β-defects (branching defects) severely degraded the photovoltaic properties of the resulting polymers and cannot be removed [41,42]. Besides the limited C-H bond selectivity, the heterogeneous palladium catalyst in strong a polar solvent DMAc also caused solubility issues in the resulting SMAs. As a result, SiliaCat ® DPP-Pd catalyzed direct arylation mainly generated the coupling products with a yield of not more than 70%.
Similar non-fused ring acceptors SMA26 and SMA27 were prepared using benzodithiophenedione (BDD) core with 2-ethylhexyl or 2-butyloctyl side chains, respectively ( Figure 10) [50]. Firstly, Pd(OAc) 2 -catalyzed direct arylation coupling was performed between BDD dibromides 48 or 49 and CPDT compound 50 to obtain intermediate products 51 and 52 with yields of around 65%. Subsequently, formyl groups were introduced by the Vilsmeier-Haack reaction at the α-position of the thiophene unit in the trimer intermediates to obtain the aldehyde-functionalized 53 and 54 in high yields. Finally, the target SMA25 and SMA26 were produced by Knoevenagel condensation with good yields of 82% and 83%, respectively. Using PM6 as the electron donor, the OSCs based on SMA26 afforded a high PCE of 12.59% with an excellent J sc of 22.57 mA cm −2 , a V oc of 0.88 V, and an FF of 63.38%. In comparison, the counterpart devices based on SMA27 showed a lower PCE of 9.80%, with J sc of 19.09 mA cm −2 , V oc of 0.87 V, and FF of 58.36%.  Similar non-fused ring acceptors SMA26 and SMA27 were prepared using benzodithiophenedione (BDD) core with 2-ethylhexyl or 2-butyloctyl side chains, respectively ( Figure 10) [50]. Firstly, Pd(OAc)2-catalyzed direct arylation coupling was performed between BDD dibromides 48 or 49 and CPDT compound 50 to obtain intermediate products 51 and 52 with yields of around 65%. Subsequently, formyl groups were introduced by the Vilsmeier-Haack reaction at the α-position of the thiophene unit in the trimer intermediates to obtain the aldehyde-functionalized 53 and 54 in high yields. Finally, the target SMA25 and SMA26 were produced by Knoevenagel condensation with good yields of 82% and 83%, respectively. Using PM6 as the electron donor, the OSCs based on SMA26 afforded a high PCE of 12.59% with an excellent Jsc of 22.57 mA cm −2 , a Voc of 0.88 V, and an FF of 63.38%. In comparison, the counterpart devices based on SMA27 showed a lower PCE of 9.80%, with Jsc of 19.09 mA cm −2 , Voc of 0.87 V, and FF of 58.36%. Figure 10. Synthetic routes of SMA26 and SMA27 by Pd(OAc)2-catalyzed direct arylation reaction [50]. . Synthetic routes of SMA22-SMA25 through Pd 2 dba 3 -catalyzed direct arylation coupling reaction [48,49].
photovoltaic properties of the acceptors were evaluated in solar cells by employing PBDB-T, PBDB-T-2Cl, and PM6 as the electron donors. The PCE of OSCs based on PBDB-T/SMA22 and PBDB-T-2Cl/SMA22 was 9.3% and 2.8%, respectively. In contrast, OSCs based on PBDB-T/SMA23 and PBDB-T-2Cl/SMA23 achieved high PCE of 10.2% and 10.5%, respectively. Moreover, the OSCs based on PM6/SMA24 and PM6/SMA25 exhibited PCE of 8.50% and 10.56%, respectively.   In addition, SMAs consisting of oligomeric cores were designed and synthesized via the one-pot direct arylation reaction, as shown in Figures 11 and 12 [51,52]. CPDT compound 50 reacted with 1,4-dibromo-2,5-difluorobenzene (55) in the presence of Pd 2 dba 3 , L1, Cs 2 CO 3 , and PivOH in toluene, to obtain the oligomers [51]. With the molar ratio of 1.6:1, 56-59 were obtained in the yield of 20.5%, 17.3%, 16.3%, and 12.5%, respectively. Through the Vilsmeier-Haack reaction and Knoevenagel condensation, the acceptors SMA28-SMA31 were produced in reasonable yields ( Figure 11). Subsequently, PBDB-T was used as the donor to evaluate the photovoltaic properties of the acceptors. With an extension of the oligomer linker, PCE of the corresponding OSCs firstly increased from 6.31% (SMA28) to 9.32% (SMA29), then gradually decreased to 5.71% (SMA30) and 2.76% (SMA31). Moreover, the J sc and FF values of the devices showed the same tendency. While benefiting from the progressively promoted LUMO energy levels, the devices' V oc values ascended from 0.77 V to 0.90 V, along with the growth of the linker lengths. the one-pot direct arylation reaction, as shown in Figure 11 and Figure 12 [51,52]. CPDT compound 50 reacted with 1,4-dibromo-2,5-difluorobenzene (55) in the presence of Pd2dba3, L1, Cs2CO3, and PivOH in toluene, to obtain the oligomers [51]. With the molar ratio of 1.6:1, 56−59 were obtained in the yield of 20.5%, 17.3%, 16.3%, and 12.5%, respectively. Through the Vilsmeier-Haack reaction and Knoevenagel condensation, the acceptors SMA28−SMA31 were produced in reasonable yields ( Figure 11). Subsequently, PBDB-T was used as the donor to evaluate the photovoltaic properties of the acceptors. With an extension of the oligomer linker, PCE of the corresponding OSCs firstly increased from 6.31% (SMA28) to 9.32% (SMA29), then gradually decreased to 5.71% (SMA30) and 2.76% (SMA31). Moreover, the Jsc and FF values of the devices showed the same tendency. While benefiting from the progressively promoted LUMO energy levels, the devices' Voc values ascended from 0.77 V to 0.90 V, along with the growth of the linker lengths. Figure 11. Synthetic routes of SMA28−SMA31 through Pd2dba3-catalyzed direct arylation reaction [51].  compound 50 reacted with 1,4-dibromo-2,5-difluorobenzene (55) in the presence of Pd2dba3, L1, Cs2CO3, and PivOH in toluene, to obtain the oligomers [51]. With the molar ratio of 1.6:1, 56−59 were obtained in the yield of 20.5%, 17.3%, 16.3%, and 12.5%, respectively. Through the Vilsmeier-Haack reaction and Knoevenagel condensation, the acceptors SMA28−SMA31 were produced in reasonable yields ( Figure 11). Subsequently, PBDB-T was used as the donor to evaluate the photovoltaic properties of the acceptors. With an extension of the oligomer linker, PCE of the corresponding OSCs firstly increased from 6.31% (SMA28) to 9.32% (SMA29), then gradually decreased to 5.71% (SMA30) and 2.76% (SMA31). Moreover, the Jsc and FF values of the devices showed the same tendency. While benefiting from the progressively promoted LUMO energy levels, the devices' Voc values ascended from 0.77 V to 0.90 V, along with the growth of the linker lengths. Figure 11. Synthetic routes of SMA28−SMA31 through Pd2dba3-catalyzed direct arylation reaction [51].  The oligomers 55-57 and 68-70, with stepwise chain lengths increasing, were obtained by one-pot direct arylation coupling between IDT (18) and BT dibromide (40) or its fluoride analog (64) with the molar ratio of 2:1 [52]. After formylation, the dialdehyde intermediates reacted with electron-deficient end unit 35 to produce the target A-Linker-A type acceptors SMA32-SMA37 ( Figure 12). The photovoltaic performance of these acceptors was evaluated by the OSCs cooperating with PM6 as the donor to investigate the influence of the π-conjugation length of the oligomeric cores. The PCE of the solar cells was 10.27% and 12.08%, for SMA32-and SMA35-based OSCs, respectively. Longer linkers sharply decreased the PCE of the acceptors to below 5% (i.e., 1.09%, 0.23%, 4.75%, and 1.43% for SMA33, SMA34, SMA36, and SMA37, respectively). This result indicated that an overlong linker weakened the D-A electron interactions and decreased the driving force for charge transfer, thus degrading the photovoltaic property of the corresponding acceptors. Increasing the feed ratio is an effective means of suppressing the formation of a higher molecular weight oligomer to improve the yield of the trimer [29]. Increasing the feed ratio of 18 to 40 (or 64) to 3:1, the reaction yields of the trimer 65 and 68 were improved to about 60% [52]. With a larger feed ratio of 10:1, the reaction yields of the crude trimers reached 83%. These results demonstrated that the oligomer distributions in this type of direct arylation could be facilely tailored by tuning the feeding ratio to obtain the target products.

Conclusions and Outlook
Direct arylation coupling reaction has produced significant advances in synthesizing πconjugated polymers and small molecules over the past two decades. This strategy reduces the synthetic steps and production cost of organic semiconductors and makes the synthesis of π-conjugated organic molecules with mild and environmentally benign conditions. This review summarized the synthesis of SMAs in OSCs via direct arylation coupling reaction. These examples demonstrate that the direct arylation coupling can be utilized to prepare a variety of SMAs through an efficient and convenient method. Thiophene and its analogs and derivatives can be effectively arylated to form aryl-aryl π-conjugation structures to construct SMAs or their intermediates. The reaction activity, selectivity, and consequent reaction yield depend on both reaction conditions and the molecular structure of reactants.
The direct arylation protocols Pd 2 (dba) 3 /P(o-MeOPh) 3 in nonpolar solvents (e.g., toluene or ODMB), have been proven to be efficient and universal in the synthesis of a broad range of SMAs. Moreover, the nonpolar solvents toluene and ODMB have much better compatibility with π-conjugated polymers than strong polar solvents, such as DMAc and DMF, to ensure dissolution of both reactants and products and restrain side reactions to the maximum extent. Until now, the studies on the synthesis of electron acceptors using direct arylation coupling have remained relatively limited. Because of the complicated synthesis process of highly efficient SMAs, there is no report focusing on the preparation of fused ring A-DA'D-A type acceptors by direct arylation coupling. Nonetheless, direct arylation coupling should be a standard replacement for more conventional Stille coupling and Suzuki coupling reactions to reduce the synthetic difficulty and preparation cost of SMAs. Hence, the synthetic strategy towards new and existed SMAs should involve direct arylation coupling to facilitate their synthesis and reduce cost. The development of direct arylation reactions in synthesizing SMAs should be essential in driving the research and application of OSCs.